The compositional analyses of blood gases and respiratory gases can provide vital information relating to the operational parameters of the several subsystems contributing to the pulmonary function, to the response to anesthetics, to therapy, and to the environment, is taught in U.S. application Ser. No. 318,152, filed Dec. 26, 1972. As disclosed in U.S. Pat. No. 3,712,111 these gases can be continually analyzed through the use of a mass spectrometer in a laboratory. Mass spectrometers are complex machines traditionally found in research laboratories where technical operator skills are readily available and frequent recalibration is not a handicap. The contribution of mass spectrometry to general pulmonary health will be enhanced if reliable operation can be achieved in a clinical environment to provide information in a continuous or on-line breath-by-breath analyses with high long-term accuracy. On-line analyses can be performed by introducing a micro-sample of the respiratory gas into the spectrometer through a controlled leak and continuously pumping to maintain the required working vacuum. This type of operation degrades the system sensitivity by causing irreversible effects in the gas ionization process, deposition of ions on the mass filter insulators, mechanical abrasion of the impact area in the ion detector, and gradual loss of detector gain. Frequent recalibration with known mixtures has been an operational necessity with systems of the prior art. In an effort to reduce the need for calibration and assure continual quality control, many gain control mechanisms have been suggested. An obvious method of gain control in a mass spectrometer is to vary the ion detector gain by varying the voltage applied to the input end of the detector. However, this introduces a mass dependent effect due to the field between the spectrometer and the detector. This is produced by the fringing field of the spectrometer and the acceleration field of the detector. The former changes with mass because of the scanning, and the latter changes with the detector gain since the voltage at the detector input is adjustable. However, variation of voltage at the exit end of the ion detector introduces additional operational difficulties such as, sample consistency, unknown temperature and pressure effects on gas, etc. Another method of gain control is to use the familiar all-ion peak of a quadrupole spectrometer, i.e., (V.sub.dc = 0) to understand its limitation.
Consider an n-component system of gases. The corresponding signals are: ##EQU1## where the subscript denotes the species, G is the detector gain, .alpha. is a constant containing the ion source parameters (electron current and source dimensions), C.sub.i is the concentration, and .gamma..sub.i involves the ionization efficiency, gas transport, transmission efficiency, detection efficiency, and pump speed.
Also, there is the constraint ##EQU2##
Assuming that .alpha. and the .gamma..sub.i 's are known, equations (1) and (2) constitute a set of n + 1 linearly independent equations in the n + 1 unknown G and C.sub.i 's. Thus, this set can be solved uniquely. In particular, the expression for the gain is: ##EQU3##
Returning Equation (1) to explain inherent limitations with an all-ion peak system, the all-ion peak control signal is calculated from the following: ##EQU4## If this all-ion peak signal is used to control the detector gain, then Sig.sub.AIP has to be a constant, i.e., EQU Sig.sub.AIP = K 5.
substituting this into equation (4) the expression for the gain is as follows: ##EQU5## and the detector signal for the i.sup.th constituent will be ##EQU6## From equation (7) it can be seen that the signal for a given molecular constituent will depend on the concentrations of the other gases present. Thus, it is not possible to use the AIP as the control signal.
Another means of stabilizing the output from the spectrometer is to use a tracer gas of "standard concentration" (C.sub.j) giving an output (Sig.sub.j) which is mixed with the sample to be analyzed. This approach is rendered impracticable by the difficulty of maintaining a "standard concentration" of the tracer gas which is independent of temperature, pressure and time.
Another method of gain control is as follows. An output signal corresponding to the j.sup.th molecular species present in the ion source would be represented as such: EQU Sig.sub.j = G .gamma..sub.j C.sub.j 8
where:
.gamma. .sub.j represents system parameters dependent on the type of gas detected, and C.sub.j is the concentration. The ratio of signals for different molecular species, say i and j, produces the following relationship: ##EQU7## Note that this ratio is independent of the signal gain, however, it is assumed .gamma., does not depend on the concentration. From Equation (9), the ratio will change with time if the concentrations change. This behavior is used in an automatic gain system in which the signal control for a given molecular species is used as the control signal, and the signal corresponding to the ratio (Equation 9) of the signals for two given species serves to abort the control change if the ratio changes. Thus, gain adjustments are made only during those periods of time in which the ratio (Equation 9) is a constant. A condition which occurs only when the concentrations remain constant. This method is impractical since the intensity of the signal is directly controlled by the stability of the contraction.